0191-9067/83 $3.00 + .00 Copyright ® 1984 SUNSAT Energy Council THE MAXIMUM EFFICIENCY OF AN ISOLATED SOLAR ENERGY CONVERSION DEVICE IN SPACE C. G. ADLER and J. W. BYRD Department of Physics East Carolina University Greenville, North Carolina 27834, USA Abstract — The conversion efficiency of radiant energy to work is considered. Interest in energy-conversion devices isolated in space has prompted consideration of optimum location of these systems to maximize the work extracted. It is found that optimizing the extraction of work is better characterized by a global efficiency defined by r)„ = W/aTjAK where W is the power extracted, T, the effective temperature of the Sun, and AK is the area of the receiver. The global efficiency has a peak which occurs at about 1.5 Sun radii from the center of the Sun. A great deal of interest has been expressed in the use of space installations for obtaining useful energy (i.e., work or the equivalent) from solar radiation. We wish to determine the maximum efficiency of such a device. In this context we define two efficiencies. First, the local efficiency given by where A,f is the area of the receiver, W is the power extracted, and Es is the solar radiation (per unit area) at the location of the device. The second efficiency is defined to be a global efficiency, i.e., where Ts is the effective solar temperature and a is the Stefan-Boltzmann constant. In the first case the location of the device is assumed to be fixed, presumably, at or near the location of the Earth. In the second case, it is assumed that the device can be located anywhere in the solar system so that is a function of space coordinates. It has been shown (1) that Es can be given by where r is the distance of the device from the center of the Sun and R is the solar radius. Written in this way, Es (for a spherical source) is exact for all values of r R. It is important to note that (R/r)2 should not be regarded as an attenuation factor, but, rather, as a view factor. If the solar radiation were to be considered to be attenuated, then one would have to conclude that its entropy increased (2); in fact, the entropy of the solar radiation is unchanged as it travels freely through space (3). The device is assumed to be operating in a steady state mode, which means that its energy and entropy cannot change. Further, it is assumed that there is no irreversible entropy production within the device. This latter condition guarantees that optimum efficiency will result (4). Since the energy of the device is constant, we must require that energy-in equals energy-out, i.e.,
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